Under a dark environment where no surrounding light is present, a user cannot recognize display on a non-light emitting display device, such as a reflective liquid crystal display device, an electrophoretic display device or an electrowetting display device, which carries out the display while reflecting surrounding light such as outside light. In order to allow a user to recognize display under such a dark environment, some of such non-light emitting display devices include, above a front surface of a display panel, a front light which illuminates the display panel.
A front light should have first and second properties below. The first property is that the front light itself is transparent so that display (a display panel) can be recognized via the front light. The second property is that (i) light emitted from the front light is mainly directed toward the display panel and (ii) less light leaks toward a user. The first property can be attained by employing a transparent light guide plate.
In order to attain the second property, a ratio of an emission intensity of light which leaks toward a user to an emission intensity of light to be emitted toward the display panel should be more than 100. The reason why the ratio should be more than 100 will be described below with reference to FIG. 22. FIG. 22 is an explanatory view for describing an emission intensity of light of a conventional reflective display device 30.
As illustrated in FIG. 22, assume that a front light 11 of the reflective display device 30 (i) emits, toward a display panel 15, light A whose emission intensity is 100 cd/m2 and (ii) emits, toward a user, light B whose emission intensity is 5 cd/m2. Assume also that (i) a reflectance of a region 16 of the display panel 15, which region 16 carries out white display, is 10% and (ii) a reflectance of a region 17 of the display panel 15, which region 17 carries out black display, is 1%. That is, a contrast between the region 16 and the region 17 is 10. In this case, light C of the light A, which light C has entered the region 16 which carries out white display, is reflected at a reflectance of 10% in the region 16 to exit as light whose emission intensity is 10 cd/m2. On the other hand, light D of the light A, which light D which has entered the region 17 which carries out black display, is reflected at a reflectance of 1% in the region 17 to exit as light whose emission intensity is 1 cd/m2. Since the light B whose emission intensity is 5 cd/m2 leaks toward the user from the front light 11, the reflective display device 30 has (i) a white luminance of 15 (=10+5) cd/m2 and (ii) a black luminance of 6 (=1+5) cd/m2. Consequently, the contrast is decreased down to 2.5. This causes a deterioration in display of the reflective display device 30.
Table 1 below shows the contrast of the reflective display device 30 in a case where an emission intensity ratio (A/B) of light A to light B from the front light 11 varies.
TABLE 1EmissionEmissionintensityintensity (B)WhiteBlackContrast(A) towardtowardluminanceluminanceofdisplaydisplayRatioof displayof displaydisplaydevicedevice(A/B) devicedevicedevice10010.01020.011.01.81005.02015.06.02.51003.33013.34.33.11002.54012.53.53.61002.05012.03.04.01001.010011.02.05.51000.520010.51.57.01000.330010.31.37.81000.340010.31.38.21000.250010.21.28.51000.1100010.11.19.2
For practical use, the contrast of the reflective display device 30 should be not less than 5. Therefore, the emission intensity ratio of light A to light B should be more than approximately 100 (see Table 1).
Patent Literature 1 discloses a technique of suppressing leakage of light from a front light toward a user. FIG. 23 is a cross-sectional view illustrating a reflective display device disclosed in Patent Literature 1. Patent Literature 1 discloses a prism-type front light 11 (see FIG. 23) in which (i) a light guide plate 19 guides light emitted from a light source 18 and (ii) prisms, formed in the light guide plate 19, direct the light toward a display panel 15 (see arrow E in FIG. 23). Patent Literature 1 also discloses a method of illuminating the display panel 15 with such light from the prism-type front light 1. However, some of the light emitted from the light source 18 certainly leaks toward a user (see arrow F in FIG. 23) depending on an angle at which the light comes into contact with the prisms. Therefore, according to the front light 11 disclosed in Patent Literature 1, it is difficult to increase, to be more than 100, a ratio of (i) an emission intensity of light to be emitted toward the display panel 15 to (ii) an emission intensity of light which leaks toward the user. This makes it difficult for the reflective display device to carry out high-contrast display.
Patent Literature 2 discloses another technique of suppressing leakage of light from a front light toward a user. FIG. 24 is a cross-sectional view illustrating a reflective display device disclosed in Patent Literature 2. Patent Literature 2 discloses a front light 11 including a light guide plate 19 on which dots 14 are formed, each of which dots is constituted by a light reflective layer 12 and a dark layer 13 (see FIG. 24). Patent Literature 2 also discloses a method of illuminating a display panel 15 with light which has been guided in the light guide plate 19 and reflected by the light reflective layers 12 (see arrow G in FIG. 24).